Press working is known as a working method wherein a part or the whole of a metal or other material is plastically deformed by using a working machine, e.g., a press, which mainly performs a reciprocating compression motion, and a die tool, thereby effecting forming, assembling, parting, straightening, etc. In the press working, the press working pressure, which is used to cause a plastic deformation to a workpiece, changes with the progress of the working process and also with changes in the atmospheric temperature or other environmental conditions. Particularly, when fine plastic working (for example, fine blanking, fine cutting, etc.) is performed by the press working, nonuniformity of pressed products occurs shortly after the start of the working process and it becomes difficult to effect stable working.
This phenomenon is explained hereinafter in more detail. In the case of a mechanical press, even if it is produced from metallic materials all of which have the same coefficient of thermal expansion, a portion of the machine body which is frequently subjected to friction heats up, and this portion alone rises in temperature. Accordingly, only the heated portion expands. As a result, the elements constituting the press are deformed. In the case of a crank press machine, for example, if the machine body expands, the position of the bottom dead center, where the pressing force is the strongest, rises, which consequently requires a lowering in the pressing force. If the ram, which lies in the center, is heated alone, it expands, and the position of the bottom dead center lowers in reverse to the above, resulting in a rise in the pressing force.
Also, in the case of a hydraulic press a phenomenon similar to that in the case of the mechanical press occurs. That is, the temperature of oil used for hydraulic pressure rises as time elapses after the start of a working process, and the pressing force rises until an appropriate temperature is reached. However, when the temperature exceeds the appropriate level, the viscosity of the oil decreases, resulting in a lowering in the pressing force.
Thus, since the change of the pressing force causes nonuniformity of pressed products, the pressing force must always be adjusted in order to realize fine working. However, even if the pressing force is set to an optimal level at the beginning of a press working, if the working process is continued for a long time, the temperature rises, and the position where pressing is effected, that is, the position of the bottom dead center, changes, resulting in a change in the pressing force.